The Third Generation Partnership Project (3GPP) initiative “License Assisted Access” (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed radio spectrum such as the 5 Gigahertz (GHz) band. The unlicensed spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum to a Primary Cell (PCell) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum by connecting to one or more Secondary Cells (SCells) operating in the unlicensed spectrum. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously used in the SCell(s) (i.e., the PCell and the SCells are synchronized). In addition to LAA operation, it should be possible to run LTE fully on the unlicensed band without the support from the licensed band. This is referred to as LTE in the Unlicensed Band (LTE-U) Standalone or MulteFire.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Because the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) scheme needs to be applied. LBT is also referred to as a Clear Channel Assessment (CCA). Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
The LBT procedure leads to uncertainty at the enhanced or evolved Node B (eNB) regarding whether or not the eNB will be able to transmit a downlink subframe(s). This leads to a corresponding uncertainty at the User Equipment device (UE) as to whether or not the UE actually has a subframe to decode. An analogous uncertainty exists in the uplink direction where the eNB is uncertain as to whether or not the UEs scheduled on the SCell are actually transmitted.